System and method of cooling steam turbines

ABSTRACT

A system and method of cooling a steam turbine having internal moving components to a predetermined temperature by controlling a flow of nitrogen through the turbine, thus decreasing the downtime associated with maintaining the turbine. This provides a more efficient and cost effective method of operating a power plant.

FIELD OF THE INVENTION

This invention relates to the field of maintenance of steam turbineshaving moving internal components, namely in the power generationindustry.

BACKGROUND OF THE INVENTION

In the power industry, electricity is produced with a spinning turbinethat is turned at high speeds to generate electricity. This turbine canbe turned by water, by gas, or by high temperature steam. The steamturbine is driven by high temperature steam from a conventional boilerreactor or nuclear reactor at speeds averaging 1800 to 3600 rpm. Many ofthe modern stream turbines operate at temperature in excess of 1,000° F.

Approximately 49 percent of the U.S. power generation in 2003 wascoal-fired and 28 percent of the generation from nuclear fuel sources.Both fossil and nuclear steam turbines experience substantial cool downtime delays associated with planned major outages, planned minoroutages, and unplanned outages. A typical steam turbine requires aminimum of one week to cool down to ambient temperatures using thecurrent methods of shutdown and outage disassembly. This inefficiencyrepresents a substantial amount of lost production and associatedrevenues for a given generating unit on an annual basis.

Power plant steam turbine metal temperatures cool down at a fairly rapidrate while steam is flowing through the turbine and the generator isconnected to the electrical grid. However, most all plant designsinclude provisions to close the steam turbine valves when the turbinesare removed from service. These design provisions are included for manyreasons, one of which is to prevent slugs of water from travelingthrough the steam turbine and causing damage to the blades and othercomponents. The technical term for this is “turbine water induction” andthe American Society of Mechanical Engineers (“ASME”) developed a steamturbine design standard many years ago after turbine water inductionfailures were reported in the industry.

When steam turbines are removed from service the emergency stop valveand the control valves remain in the closed position. This restrictssteam flow through the steam turbine and results in a “bottled up” steamturbine. Typical steam turbines with weights of 125 to 200 tons andoperating temperatures of 1000° F. have significant thermal mass. Afterremoving the steam turbines from service, the shell and rotortemperature remains above 700° F. at many locations while the turbine is“bottled” up for many days. Even after one week of conventional coolingdown methods, it is not uncommon to measure temperatures in excess of150° F. on the rotor and thick shell locations.

The prior art fails to provide for a cooling down large steam turbinesat electrical generation stations. Steam turbine sizes increased rapidlyfrom 1950 to 1970. During this period, manufactures focused onminimizing rotor stresses and reducing large temperature changes to thatresult in shell cracking. Moreover, the prior art failed to minimizeturbine outages to the extent that commonly exists today. Therefore,there exists a need to cool down steam turbines and to optimize outagetime.

The prior art has approached these problems by pre-staging andmobilizing while the turbines are in a cooling down period. Generally,the cool down period in the prior art is concurrent with the disassemblyof the steam turbine and occupies the first week of most outages. Inrecent years the electric utility industry has attempted to reduce thecosts associated with turbine outages. The prior art has focused onrapid disassembly and repair techniques while accommodating this cooldown period. Therefore, a need exists to improve and optimize outagesassociated with steam turbines in the electric utility industry thatwould offer cost incentives associated with electric productionincluding replacement power costs, labor costs, repair costs, and plantoperating availability requirements.

There are many factors that attribute to the cool down time of a steamturbine. When turbines are taken off-line or removed from service,electrical breakers are opened, thus removing the generators from theelectrical grid. Next, the main steam stop valve and control valves shutautomatically to prevent damage due to water induction. The turbine isde-pressurized minutes after shut-down but the steam turbine is ineffect “bottled up” as it related to metal temperature. There is nosubstantial cooling fluid available internally and the turbine coolsvery slowly as heat escapes only through the shell and outer insulation.After a steam turbine is removed from the grid, the turbine is allowedto spin freely from its nominal operating speed down to approximately 90rpm in twenty to thirty minutes.

During the spin-down there is usually only minimal steam flow in thereverse direction through the high pressure turbine to prevent excessivetemperatures due to steam stagnation. The lube oil system remains inoperation while the “hot” turbine is on turning gear until the firststage metal temperature reaches approximately 500° F. This takesapproximately 40 to 80 hours depending on the turbine. At this time theoil system can be taken offline and the turbine can be completelydisassembled. The initial outer and inner shells are often removed withsome component temperatures at several hundred degrees Fahrenheit. Theturbine rotor usually is removed and placed on the stands near theturbine with internal temperatures exceeding 150° F. Most high pressureturbine rotors are not physically removed from the inner shell casinguntil five to eight days after the unit is taken off-line.

Though there is no consistent method of turbine disassembly, the turbineis disassembled hot and cooling occurs after disassembly in the priorart. The current practices are followed to reduce the metal temperatureswithout damage to the turbine. In the prior art, cooling down theturbine assembly can only occur as long as steam flows through theturbine.

Because these methods require workers to disassemble the turbine priorto the cool down process, cool down the turbine and disassembly mayoccur at temperatures between 200 and 500° F. The rate of turbine cooldown depends on the willingness of workers to work on hot components,safety concerns, rigging limitations, and insulation removal activities.On large steam turbines, meaningful turbine cooling of the shell isusually not achieved until the crossover pipe between the high pressureturbine section and/or intermediate pressure turbine sections and thelow pressure turbine section is removed. This is usually not attempteduntil the turbine's metal temperatures are less than 600° F. Once thecrossover pipe is removed, the rate of cooling due to air convectionincreases dramatically. In summary, this method does not providereasonable, efficient, or adequate cooling for outage and costoptimization.

The prior art offers two methods to improve the cooling time associatedwith large steam turbines. The primary method of reducing the cool downtime is to force cool the turbine components prior to bringing theturbine off-line. This typically consists of reducing the main steamtemperature just prior to removing the steam turbine from service. Themain steam temperature is reduced by closing the extraction steamsources to the feedwater heaters, reducing steam temperature throughattemperation, and slowly lowering the steam temperature to nearsaturation temperatures. This forced cool down method is expected to beused on 400 megawatt and larger units.

Though this method of forced cool down removes a substantial amount ofheat from the steam turbine, the saturation temperature limitations andpotential for water induction fails to provide for a substantial cooldown of the steam turbine. This method only cools the turbine down fromover 1000° F. to temperatures between 700-900° F. in the high pressureturbine section of the turbine. This only saves about one to about oneand one half days in the unit's outage shutdown.

Moreover, this method offers the additional problem that even after thisforced cool down, the operator cannot shut off the lubrication system onthe turbine and the operator cannot disassemble the turbine until themetal temperatures are between 500-600° F. Furthermore, there is asignificant hazard with disassembling the turbine at this hightemperature.

In addition, using this cool down method while the unit is still inservice may fail because the turbine operations will trip for severalreasons. Therefore, this method of forced cool down is unable to offeran efficient method of cooling down the turbine to allow maintenance ofthe turbine. Moreover, the calibration of station instrumentation, thecondition of level detection devices, the inadequate operational staffduring shutdown, the system power levels and load changes, the boilertuning, and the other plant configuration matters will further limit theusefulness of this method.

The second method of the prior art is air cooling. Various air horns areplaced at limited internal access points throughout the turbine. Thismethod does not provide efficient cool down of the turbine because airat near ambient pressure does not provide enough heat transport andnon-uniform distribution of cooling. This method results in humping,which is stagnant steam distributing in large turning shells in a mannerthat stratifies the steam and results in convective heating that iscooler on the bottom and warmer on the top. This problem occurs when theturbine is “bottled up” with hot steam and the metal is slightly longerat the top of the shell than at the bottom of the shell. Thismisalignment can cause rotor contact.

This method is also deficient because of limited accessibility, limitedcompressed air capacity, and non-uniform distribution of coolingthroughout the turbine. These three factors result in not only a lack ofcooling capacity but problems with shell humping and non-uniformcomponent cooling. Moreover, two limitations of forced cooling using airinjection are adequate cooling capacity and uniform cooling. This methodhas only been shown to save minimal time in the shut down of steamturbines.

The electric utility industry has experienced dramatic changes over thepast two decades and continues to encounter significant competitivechanges associated with the generation of electricity. These competitivechanges are a driving force to produce electricity more efficiently andcost effectively. Electric utility power plant outages are an integralpart of the electric power industry and therefore are a criticalcomponent in evaluating electrical demand and the ability to satisfy thedemand through generation resource allocation.

These outages consist of both planned and unplanned events that havevariable time durations depending on various factors. The costconsequences of outages is highest during summer and winter peak powerdemand periods, whereas outage costs during other time periods are morepredictable. Other factors that influence outage consequences includereduced electric capacity, power plant age, wholesale market price andvolatility, environmental regulations, electric deregulation, and otherfactors. With respect to power plant age, it is noted that the averageage of most of the large power plants in the U.S. is over 30 years whichemphasizes the importance of regular scheduled maintenance outages whichin turn requires time and money. Therefore, a need exists to minimizethe costs associated with an outage by reducing the amount of timeneeded to service the turbines.

Flows of nitrogen have been used in different arts, but this technologyhas not been implemented on devices with internal moving parts. In thechemical, petrochemical and oil refining industry, various reactorvessels that operate at 1,000° F. are taken off line for maintenance.These processes are used to reduce crude oil to useable end products.These large reactor vessels contain various catalysts that aid the crudeprocessing. These catalysts become spent and are required to beperiodically replaced. The reactor vessels must be cooled down fromtheir operating temperatures over 600° F. to less than 100° F. Theprocess equipment being cooled in this art are reactor vessels that arestationary and static with no moving parts. Care must be taken to notcool the metal to-too fast that can cause metal fatigue and crackingfrom stresses caused from shocking the metal. For vessels with no movingparts, liquid nitrogen has been forced through vessels having metalsurfaces at greater than 350° F. Most metal can be cooled down at ratesof 75-100° F. per hour.

In the early 1980s, Union Carbide Industrial Services used liquidnitrogen to cool down reactor vessels instead of recycling processnitrogen and hydrogen gases through the units compressor systems. Inoperation, the plant systems allowed recycling the gas through thereactor to absorb heat, and then passing that gas through the system'sheat exchangers to cool the gas. The compressors pumped the gas backthrough the reactor to force cooling. This required four to six days toobtain temperatures below 100° F.

In contrast, a steam turbine operates under very dramatic conditionswith a large spindle spinning at 3600 rpm inside a stationary shell. Theheat shrinkage of the stationary shell to the spindle and the stationaryshell to the spindle blades of the turbine is a factor. If the coolingis not completed with careful control so that all parts of the machinerycool at the same rate, damage can occur to the machine with spinningparts coming into contact with stationary parts, humping occurring, andthe weight of the turbine shifting from one end to the other too fast,thus causing damage. Therefore, a need exists to provide for a cool downof a turbine with moving internal parts such that the cool down rate maybe controlled.

SUMMARY OF THE INVENTION

The present invention offers a cool down method applicable to rotatingand moving equipment in the electric power generating industry includingsteam turbines. By using a flow of nitrogen, preferably a forced flow ofnitrogen, the present invention may improve the efficiency of powerplant outages by reducing the cool down time associated with large steamturbines. This cool down period can represent a substantial cost if theoutage occurs at critical electrical power market conditions. This costbecomes much more substantial and can result in extreme costs if themarket conditions increase over $50 per megawatt.

Nitrogen is pumped as a gas into a moving steam turbine machine in acontrolled manner. This method prevents stress cracking the turbinesmetals without causing the machine to warp, humping, or have unevencooling across the machine that could cause moving parts to come intocontact with non-moving parts. This invention is also capable ofpreventing overspeeding of the blades of the turbine above their designspeed.

The present invention also allows for faster shut down for cleaning.Power plants shutdown and need to cool the turbine for many reasonsincluding cleaning. Prior to cleaning, the steam turbine metaltemperature may need to be below 175° F. Therefore, the presentinvention provides for greater opportunity to clean the internalcomponents of the turbine. In the preferred embodiment, a steam turbinecoming off production will have a temperature profile of 500-1,000° F.across the machine.

The present invention provides a flow of nitrogen from a nitrogen pumperto a flow control station installed under the steam turbine. Thenitrogen pumper pumps the nitrogen through a single piping header intothe area under the steam turbine. The gas is divided into different flowstreams in a nitrogen flow control station, which is designed to controlthe nitrogen flow and temperature being delivered to different sectionsof the turbine.

The present invention may also take advantage of the many differenttypes of instrumentation that already exist on the turbine to monitortemperature, vibration, and growth or shrinkage of the machine as itoperated. As the nitrogen gas is introduced into various ports orconnections on the steam turbine, these instruments are monitored acrossthe machine to monitor how the machine is reacting to the rate that thenitrogen is being introduced. The ports or connections used on theturbine for nitrogen injection will depend on the various designs thatexist in the power industry today.

Therefore the different methods discussed herein provide options forapplying the nitrogen without damaging the internal, moving componentsof the turbine by uneven cooling, rapid cooling, or over-spinning. Withthe nitrogen flow control station, nitrogen can be introduced todifferent areas of the turbine at different temperatures and/ordifferent flow rates and cooling or heating can be accomplished atdifferent rates in different areas of the turbine so that the machine iscooled down evenly without damage.

The force cooling of power plant steam turbines is significantlydifferent than the cooling down of process equipment. The rate of metalcontraction in a large steam turbine is significant. A steam turbine isa large piece of equipment that is designed to grow and shrink over afoot in length as it goes on and off production. The growth of the steamturbine varies with size from the simple equation:ΔLength=αLength of steel turbine casing×ΔTemperature in ° F.αis the coefficient of thermal expansion for a given material and is inthe 10⁶ range. Therefore, the typical growth is from about two inches toabout 12 inches on a very large turbine.

Because the clearance tolerance between the casement and the spindle isso tight, the rate of shrinkage of the different parts of the machine isimportant to the cool down process. Thus the control of the cool downtemperatures is significant. It is envisioned that the present inventionwill be able to reduce the cool down period from the five to eight daysof the previous methods to less than approximately two days, preferablyless than about 36 hours. This invention also offers a costs saving byproviding a quicker shutdown or turnaround time and by extending theproduction of electricity during the cool down process.

The present invention may be incorporated into or used on a variety ofturbines via a variety of turbine connections that are differentdepending on the manufacturer of the turbine. The present invention isdescribed in conjunction with one embodiment of the invention, but thoseskilled in the art recognize that the teachings herein are equallyapplicable to different embodiments with varying connections.

In the preferred embodiment the present invention provides a method ofcooling a steam turbine having internal moving components to apredetermined temperature, wherein the steam turbine comprises a mainsteam inlet piping connected to the turbine and a cold reheat lineconnected to the turbine such that a flow of steam first moves from themain steam inlet piping to the turbine and then the flow of steam movesfrom the turbine to the cold reheat line during operation, the methodwhich includes the steps of stopping the flow of steam, introducing aflow of nitrogen to the turbine until the turbine reaches thepredetermined temperature while controlling the flow of nitrogen toprevent damage to the moving components of the turbine, and stopping theflow of nitrogen. This method may also include a hot reheat lineconnected to the turbine and a condenser vacuum relief line connected tothe turbine such that a flow of steam first moves from the hot reheatline to the turbine and then the flow of steam moves from the turbine tothe condenser vacuum relief line during operation, wherein the flow ofnitrogen in also moves from the hot reheat line to the turbine and thenthe flow of nitrogen moves from the turbine to the condenser vacuumrelief line.

The method may also include a main steam inlet piping drain lineconnected to the main steam inlet piping, the method wherein the flow ofnitrogen moves from the main steam inlet piping drain line to the mainsteam inlet piping. In a preferred embodiment, the method includes acold reheat drain pots connected to the cold reheat line, the methodwherein the flow of nitrogen moves through the cold reheat drain pots tothe cold reheat line. Each of these methods benefits from controllingthe flow of nitrogen with a computer control system.

The present invention also includes a system for cooling a steam turbineto a predetermined temperature using a flow of nitrogen, the systemincluding a steam turbine, a main steam inlet piping connected to theturbine, a cold reheat line connected to the turbine, and a controlstation for controlling the flow of nitrogen to prevent damage to themoving components or the turbine wherein the steam turbine, the mainsteam inlet piping, and the cold reheat line are adapted to accommodatethe flow of nitrogen. This system may also include a hot reheat lineconnected to the turbine and a condenser vacuum relief line connected tothe turbine wherein the hot reheat line and the condenser vacuum reliefare adapted to accommodate the flow of nitrogen. In another embodiment,the system may also include a main steam inlet piping drain lineconnected to the main steam inlet piping wherein the main steam inletpiping drain line is adapted to accommodate the flow of nitrogen. Thisinvention may also include cold reheat drain pots connected to the coldreheat line adapted to accommodate the flow of nitrogen. The controllerof this system is a computer control system wherein the computer controlsystem is adapted to control the flow of nitrogen in a preferredembodiment.

This invention also provides for a more efficient and cost effectivemethod of operating a power plant by using the methods listed above toreduce downtime by cooling each steam turbine using the inventiveconcepts as disclosed and claimed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiments of the present invention,and, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 is diagram including a cross sectional view of an embodiment ofthe present invention;

FIG. 2 is diagram including a cross sectional view of an embodiment ofthe present invention;

FIG. 3 is diagram including a cross sectional view of an embodiment ofthe present invention;

FIG. 4 is diagram including a cross sectional view of an embodiment ofthe present invention; and

FIG. 5 is diagram including a cross sectional view of an embodiment ofthe present invention.

It is to be noted that the drawings illustrate only typical embodimentsof the invention and are therefore not to be considered limiting of itsscope, for the invention encompasses other equally effectiveembodiments. Nitrogen injection points will vary due to the differentdesigns and piping configurations that exists on the existing turbine inthe power production market.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The present invention offers forced cooling of a flow of nitrogen suchthat a steam turbine may be cooled in a shorter period of time. In amost preferred embodiment, it is expected that a typical large fossilsteam turbine unit between 200 and 850 megawatt (“MW”) may be cooled inless than about 48 hours, more preferably less than about 36 hours, mostpreferably about 24 hours. In fact, it is envisioned that a cool downcould be accomplished in as little as about four to about 15 hours in amost preferred embodiment. The time required is dependent on the overallmass of the turbine and the nitrogen flows that can be obtained. Thecooling time will be determined by the difference between the time ofoperation and the time at which the turbine has cooled down from therange of about 350° F. to about 700° F. to a predetermined temperaturesuch as less than about 100° F. to about 200° F. It is expected thatsubstantial differences between turbine sizes, manufactures and powerplant system will exist and therefore affect this cooling time.

In another embodiment, the present invention may be used for pre-heatingof the steam turbine as well. This may result in some start-up timesavings of about 4 to about 40 hours, depending on the unit consideredand the type of power plant start-up performed. Once this technology isimplemented in a plurality of locations, there will be an overallcapacity increase of U.S. electric power generation for NationalElectric Reliability Council (“NERC”) regions or for individualutilities. After studying the outage schedule produced for each NERCregion and an expected outage time savings for the combined powerplants, it is likely that being able to put power plants back on-linesooner by the present invention may result in more overall electricenergy produced by the U.S. as a whole or by a separate region. Thepresent invention may also be used in combination with othertechnologies. For example, nitrogen injection may be used on other powerplant equipment for purposes of corrosion prevention. In a preferredembodiment, the present invention includes a braking device to allowcontrolling the turbine rotation speed at a turning gear speed. Thesedesigns would address the need to not over spin the turbine with thenitrogen inject and preferably maintain a controlled and constantrotation speed less than operational design spin but sufficient tofacilitate the flow of nitrogen through the machine. This device wouldbe installed on the turbine/generator shaft to control the speed of theturbine-to-turbine gear speeds. This provides enough torsionalresistance to maintain the turbine at turning gear speeds and reduce thedamage to the turbine in the event that the turbine rotor expands orcontracts relative to the shell.

FIGS. 1-5 show several different embodiments of the present inventionwith respect to a steam turbine. Those skilled in the art will recognizethat the concepts disclosed and taught herein are applicable to steamturbines and similar with internal, moving elements. Nothing herein isintended to limit this teaching to any particular type of steam turbine.The inventive aspects of using the injection points of steam turbines inthis manner is equally applicable to any manufacturer's turbines.

This steam turbine 10 features an opposed flow high pressure turbinesection 12/intermediate pressure turbine section 14 on a single turbine10 and an opposed flow low pressure turbine section 16 on a single rotor18. This design is exemplary of the large steam turbines in the UnitedStates and around the world. This turbine 10 is typical of thematerials, weights, sizing, rotors, and shells of other makes andmodels. Those skilled in the art recognize that variations will existdepending on the size of other steam turbines, shell thickness, flowconfiguration, manufacture, and power rating.

FIGS. 1-5 show different arrangements of flow paths for the flow orflows of nitrogen throughout the turbine 10 for the purpose of cooling.These flow paths are examples of the methods of cooling a steam turbine.Of note, the direction of the flow of nitrogen, the size and capacity ofthe piping, the heat load stratification of steam turbine shells androtor, the location of nitrogen injection, the location of nitrogendischarge, the confined space and oxygen depravation considerations, theflow capacity of nitrogen, and the cooling transport affect theteachings of the invention.

The path of the flow of nitrogen through the turbine 10 is a focus ofthe invention. The forced cooling can be accomplished in any path/flowdirection. However, the steam turbine 10 will cool at a faster rate ifthe flow of nitrogen is injected on the cooler side of the turbineshell, shown as outer shell 20 and inner shell 22, and dischargednearest the location of the highest turbine shell metal temperature.This provides the highest nitrogen-to-metal mismatch temperaturethroughout the steam turbine 10.

The second item to consider related to the nitrogen flow direction isthe speed of the steam turbine 10. The “free spinning” of theturbine/generator set affects the cooling time of steam turbines. Thesteam turbine is by design a nearly frictionless mass of rotatingmachinery. The turbine system is designed to reduce rotational losses inan effort to reduce power losses, reduce bearing 24 size, and generallyincrease the overall efficiency of the steam turbine 10. The bearings 24of a large steam turbine are an important critical machined componentand provide a near frictionless surface.

Typically all bearings 24 in a steam turbine 10 are babbit bearingsincluding the thrust bearings. All bearing surfaces of modem steamturbines include forced lubrication and thermocouples for precisemonitoring of bearing temperatures. Therefore, the flow of nitrogen willhave direct relationship with speed. In some embodiments, the flow ofnitrogen is in the opposite direction to counteract the rotational speedeffect of the blades 26 of the turbine 10 created by the flow ofnitrogen.

The turbine 10 shown in FIGS. 1-5 is an opposed flow design. Steamturbines are designed to operate at normal speeds up to 3600 rpm whileavoiding certain turbine speeds associated with resonant turbine rotorfrequencies. Precise turbine rotor frequency ranges are available fromthe manufacture and will vary with design.

During the cooling process, the vibration levels associated with thespeed of the blades 26 of the turbine 10 are not substantially differentwith nitrogen than steam. Therefore, it is important during nitrogenforced cooling that the turbine speed needs to be less than 3600 rpm andoutside the vibration frequencies for a given turbine.

Nitrogen forced cooling can be accomplished at various speeds, dependingon the desired cooling flow. Rotor 18 speed may increase rapidly if theflow is increased or unbalanced suddenly. Therefore, it is important tonot overspeed during operation and during cool down to prevent permanentdamage to the rotor 18 and blades 26. Therefore, it is important to havea device or method of controlling the flow of nitrogen to preventoverspeed conditions.

The size of the piping connections, such as the main steam inlet piping28 and the cold reheat line 30 shown in FIGS. 1-5, and their capacitylimitations are important to determine the nitrogen cooling capacity.Most methods will be generally limited to the existing standard pipingavailable at the steam turbines.

Because steam turbines transfer work in the form of steam energy tomechanical machine inertia or torque, the steam pressure and temperaturein the form of heat energy or enthalpy is reduced in the process offlowing through each set of turbine blades during operation. Therefore,there is stratification from about 1000° F. to about 600° F. for mosthigh pressure turbine sections and intermediate pressure turbinesections.

Therefore, the present invention benefits from producing a flow ofnitrogen in the same direction as an increase in metal temperature. Thisprovides cooling flow through the steam turbine 10 and minimizes thenitrogen gas to metal mismatch temperature. The nitrogen to metalmismatch temperature and its associated steam to metal mismatchtemperature may be a surface metal stress limit for the turbine shell 22and the rotor 18. Steam turbines are designed in accordance withpublished steam to metal mismatch temperatures. The present inventionprovides for cooling while staying under the stress limits published bythe turbine manufacture.

Moreover, the location of injection or introduction of the flow ofnitrogen is important. Factors including the size of existing piping orconnections, the length of piping runs, the location on the turbine, theease of connection, the proximity to the nitrogen pump truck, and theturbine metal temperature should be considered.

The location of the nitrogen discharge is also important. Factors toconsider include confined space safety, oxygen depravation, transport toatmosphere, and existing steam turbine and power plant piping. Ingeneral, it is preferable to accommodate the location of existing pipingand connections and the location for discharge to atmosphere.

Turning to the confined space and oxygen depravation considerations, theuse of large volumes of nitrogen in a power plant may require specialconsideration of confined space requirements for a given power plant andutility. It is important to vent the nitrogen in a manner that will notcreate an oxygen depravation issue in a confined space.

The cooling rate of the steam turbine is primarily influenced by theamount of flow of nitrogen through the turbine given that the heatcapacity and temperature differential will be affected by a giventurbine design and operating condition. In the present invention, thespeed of the blades 26 of the turbine 10 will be the notable factorassociated with the maximum pounds of the flow of nitrogen that can becontrolled.

Focusing on FIG. 1, the turbine 10 includes an introduction of the flowof nitrogen at the main steam inlet piping drain lines 32 connected tothe main stream lines 28. The flow of nitrogen moves from the main steaminlet piping drain lines 32 to the main stream lines 28 and then to theturbine 10. The flow of nitrogen then moves from turbine 10 to coldreheat line 30 with an exhaust at the cold reheat safety relief valve onthe boiler roof (not shown). This method provides for nitrogen forcedcooling with minimal piping connections and plant impact.

It is recognized that the main steam inlet piping drain lines 32 shouldbe in-service service to conform to the ASME guidelines for preventionof water induction. However, once the main stop valve (not shown) isclosed and blades 26 of the turbine 10 are placed on turning gear, thispiping system is available for use in the cool down process of thepresent invention.

The flow direction in this embodiment is reverse of the more efficient,forward direction and will be limited by the steam turbine surface metalto nitrogen gas mismatch temperature. This method also includes thecooling of the high pressure turbine section 12 only. There is nonitrogen flow through the intermediate pressure turbine section 14 andlow pressure turbine section 16 turbine sections. The cooling of theintermediate pressure turbine section 14 and the low pressure turbinesection 16 will be through conductive means. As the high pressureturbine section 12 and rotor 18 cool, the heat flux moves in thedirection of the high pressure turbine section 12, essentially pullingheat from the intermediate pressure turbine section 14 and the lowpressure turbine section 16. The advantage of this embodiment is therelative simplicity and ease of implementation on existing turbinesystems.

Turning to FIG. 2, the turbine 10 is shown in another embodiment. Thisembodiment includes additional piping connections: a hot reheat line 34and a condenser vacuum relief line 36. This system and the method of itsuse include the same elements as shown in FIG. 1, but include a secondflow path through the intermediate pressure turbine section 14 anddownstream low pressure turbine section 16. The flow of nitrogen flowsfrom the hot reheat line 34 to the turbine 10 and from the turbine 10 tothe condenser vacuum relief line 36.

Both the high pressure turbine section 12 and the intermediate pressureturbine section 14 have flow directions in the less efficient direction,referred to herein as forward. However, this efficiency is balanced bythe simplicity and relatively low-cost for implementation of thisdesign. It is important to monitor and control the speed of the blades26 of the turbine 10 in this embodiment.

Referring to FIG. 3, an embodiment of the present invention is shownsuch that the flow of nitrogen is in the reverse direction through thehigh pressure turbine section 12 only. The flow of nitrogen is injectedat the cold reheat line 30 from cold reheat drain pots 38 controlled bythe controller 40. The flow of nitrogen moves from the cold reheat line30 through the turbine 10 and from the turbine 10 to the main steaminlet piping 28.

The flow of nitrogen is then discharged to atmosphere through the blowdown valve 42. Modem blow down valve 42 piping can either flow to thecondenser or the roof in most cases. The roof is simpler and does nottypically require piping modifications.

This embodiment is the first to show a controller 40, but the advantageof using a controller to monitor and moderate the flow of nitrogenthrough the turbine 10 is an important aspect of the invention. Thecontroller 40 is adapted to control the flow of nitrogen to preventdamage to the moving components or the turbine, including unevencooling, rapid cooling, or over-spinning.

Turning to FIG. 4, another embodiment is shown. This system and methodprovide a flow of nitrogen to the high pressure turbine section 12 inthe reverse direction and a forward flow of nitrogen to the intermediatepressure turbine section 14 and the low pressure turbine section 16.This provides a higher overall nitrogen gas flow at a lower turbinespeed due to the counteracting effect of the work or torque on the highpressure turbine section 12 opposing the intermediate pressure turbinesection 14 and the low pressure turbine section 16.

In this embodiment, the speed of the blades 26 of the turbine 10 aremaintained by the flow of nitrogen managed by the controller 40 betweenimportant rotor frequencies less than 1000 rpm. The controller 40adjusts two nitrogen admission valves 44 to accomplish a higher flow ofnitrogen through the turbine 10. This embodiment may provide flows ofnitrogen greater than 7500 lbs/hr and the cooling may only be limited bymaximum shell stress conditions on most units.

This method features controller 40 that includes a computer dataacquisition and control system to coordinate the nitrogen admissionvalves 44, turbine speed, flow of nitrogen, turbine shell 20 and 22temperatures, turbine rotor 18 temperatures, first stage metaltemperature, axial shell to rotor clearance, and needed information asapplicable. The controller 44 is equally applicable to any embodiment ofthe present invention. The controller 44 also avoids turbine over speed.

A combination of additional field equipment and additional projectprocedures are helpful in preventing overspeed condition of the turbine10. The additional field equipment may include nitrogen stop valvesupstream of the control valves 44 for nitrogen admission. The stopvalves are preferably the “fail close” design for rapid valve travel toprevent overspeed condition of the turbine. This system and method is amost preferred embodiment for the reasons stated herein.

Referring to FIG. 5, an arrangement of the system and method similar tothe design in FIG. 1 is shown, but with different nitrogen gasconnection points. This system and method includes nitrogen gas inlet 46through the main steam inlet piping 28 such that the flow of nitrogenmoves from the main steam inlet piping 28 to the turbine 10 and from theturbine 10 to the cold reheat line 30. From the cold reheat line 30, theflow of nitrogen moves through the cold reheat drain pots 38 and isvented to the atmosphere. This embodiment may be a consideration forturbine cooling using the main steam inlet piping 28 as a method ofnitrogen gas injection.

Another aspect of the present invention is the economic efficiencies andthe benefits of the present invention. A modem steam generating plantproduces electricity at a cost of $18.00 US to $35.00 US per megawatthour (“MW/hr”) of operation. A nuclear powered plant produceselectricity at a cost of about $18.00 US per MW/hr. A large coal plantproduces electricity at a cost of about in the $23.00 to to $28.00 rangeand a gas plant produces electricity at a cost of about $25.00 to $30.00range. Older less efficient coal, oil or gas fired plants or gas firedturbines produce electricity at a cost that can be as high as $40 to$45.00 per MW/hr.

The time required to cool down a given steam turbine depends on its sizeand the operating temperatures and pressures. A nuclear plant turbine isthe largest in physical size, but lower in operating temperature andpressure and usually in the megawatt capacity of more than about 1,000MW. These units require approximately ten days to cool down byconventional methods. Fossil fired generating units using coal, oil, orgas can be as large as a nuclear steam turbine, but most units operateat higher pressures and temperatures. Most of the fossil units are inthe about 400 to about 600 MW capacity size. The cool down of thesmaller fossil units using conventional methods are from about five toabout seven days.

When a plant is offline, the lost production cost can be much higherthan the cost of operating the plant if the electrical demand is up. Anuclear plant can produce power at $18.00 per MW/hr, but the company mayhave to purchase power on the power grid that is generated by gasturbines that can cost as much as $45.00 per MW/hr. The followingprovides the daily operating costs and potential savings associated withthe use of the present invention.

TABLE 1 Daily Operating Costs and Potential Savings Costs & PotentialType of Daily Operating/ Savings/ Daily Plant MW Size Product CostCost/Savings Nuclear 1000 $18.00 $432,000 Fossil 1000 $25.00 $600,000Fossil 800 $25.00 $480,000 Fossil 600 $25.00 $360,000 Fossil 400 $28.00$268,000 Fossil 200 $30.00 $144,000This table shows that each day that can be saved in cooling down a giventurbine, dependent of MW size and MW cost of product, can save a powerplant from $144,000 to $600,000 per day. In a nuclear plant, that couldbe a cost saving amounting to as much as $3 M in one shutdown for sevendays. On an average size 600 MW plant, the saving could also be as highas $3 M, with a smaller power production unit savings being as little as$0.5 M. This shows that the production costs savings can be verysubstantial with a very high “value added” worth to this service to agiven power plant operation.

Having described the invention above, various modifications of thetechniques, procedures, material and equipment will be apparent to thosein the art. It is intended that all such variations within the scope andspirit of the appended claims be embraced thereby.

1. A method of cooling a steam turbine having internal moving componentsto a predetermined temperature, wherein the steam turbine comprisesinjection points, the method which comprises the steps of: (a) stoppingthe flow of steam; (b) introducing a flow of nitrogen to the turbineuntil the turbine reaches the predetermined temperature whilecontrolling the flow of nitrogen at the injection points to preventdamage to the moving components of the turbine by achieving uniformcooling of the internal moving components; and (c) stopping the flow ofnitrogen wherein the injection points comprise a main steam inlet pipingconnected to the turbine and a cold reheat line connected to the turbinesuch that a flow of steam first moves from the main steam inlet pipingto the turbine and then the flow of steam moves from the turbine to thecold reheat line during operation, wherein the steam turbine furthercomprises a hot reheat line connected to the turbine and a condenservacuum relief line connected to the turbine such that a flow of steamfirst moves from the hot reheat line to the turbine and then the flow ofsteam moves from the turbine to the condenser vacuum relief line duringoperation, and wherein the flow of nitrogen in also moves in the hotreheat line during Step (b).
 2. The method of claim 1, wherein the flowof nitrogen moves from the main steam inlet piping to the turbine andthen the flow of nitrogen moves from the turbine to the cold reheat lineduring Step (b).
 3. The method of claim 1, wherein the flow of nitrogenmoves from the cold reheat line to the turbine and then the flow ofnitrogen moves from the turbine to the main steam inlet piping duringStep (b).
 4. The method of claim 1, wherein the flow of nitrogen in alsomoves from the hot reheat line to the turbine and then the flow ofnitrogen moves from the turbine to the condenser vacuum relief lineduring Step (b).
 5. The method of claim 2, wherein the flow of nitrogenin also moves from the hot reheat line to the turbine and then the flowof nitrogen moves from the turbine to the condenser vacuum relief lineduring Step (b).
 6. The method of claim 3, wherein the flow of nitrogenin also moves from the hot reheat line to the turbine and then the flowof nitrogen moves from the turbine to the condenser vacuum relief lineduring Step (b).
 7. The method of claim 2, wherein the main steam inletpiping comprises a main steam inlet piping drain line connected to themain steam inlet piping, the method wherein the flow of nitrogen movesfrom the main steam inlet piping drain line to the main steam inletpiping during Step (b).
 8. The method of claim 7, wherein the flow ofnitrogen also moves from the hot reheat line to the turbine during Step(b).
 9. The method of claim 3 wherein the turbine further comprises acold reheat drain pots connected to the cold reheat line, the methodwherein the flow of nitrogen moves through the cold reheat drain pots tothe cold reheat line in Step (b).
 10. The method of claim 3 wherein theturbine further comprises a cold reheat drain pots connected to the coldreheat line, the method wherein some of the flow of nitrogen movesthrough the cold reheat drain pots to the cold reheat line in Step (b).11. The method of claim 3 wherein the turbine further comprises a coldreheat drain pots connected to the cold reheat line, the method whereinthe flow of nitrogen moves through the cold reheat line to the coldreheat drain pots in Step (b).
 12. The method of claim 1, the methodwhich further comprises the step of controlling the flow of nitrogenduring Step (b) with a computer control system.
 13. The method of claim1, which further comprises introducing a heated flow of nitrogen to theturbine to preheat the internal moving components of the turbine afterStep (c).
 14. A system for cooling a steam turbine to a predeterminedtemperature using a flow of nitrogen, the system comprising: a steamturbine; a main steam inlet piping connected to the turbine; a coldreheat line connected to the turbine; and a control station forcontrolling the flow of nitrogen to prevent damage to the movingcomponents or the turbine; a hot reheat line connected to the turbine;and a condenser vacuum relief line connected to the turbine; wherein thehot reheat line and the condenser vacuum relief are adapted toaccommadate the flow of nitrogen; and wherein the steam turbine, themain steam inlet piping, and the cold reheat line are adapted toaccommodate the flow of nitrogen.
 15. The system of claim 14 furthercomprising a main steam inlet piping drain line connected to the mainsteam inlet piping wherein the main steam inlet piping drain line isadapted to accommodate the flow of nitrogen.
 16. The system of claim 14further comprising a cold reheat drain pots connected to the cold reheatline adapted to accommodate the flow of nitrogen.
 17. The system ofclaim 14 further comprising a computer control system wherein thecomputer control system is adapted to control the flow of nitrogen. 18.The System of claim 14 wherein the predetermined temperature is lessthan about 200° F.
 19. A method of providing a more efficient and costeffective method of operating a power plant, the method which comprisesthe Step of reducing downtime by cooling each steam turbine using themethod of claim 1.